Just install the plugin and make a Gravity Form with a List Field. You will see this extra options:

You can translate this plugin by using the .po file. Download it as a zip here below. Based on version 1.3.3 of this plugin.

Upload the translation files in this folder on your server:

wp-content/languages/plugins/

So, for the Dutch (Nederlandse) translation for example, it should look like this on your server:

wp-content/languages/plugins/list-field-number-format-for-gravity-forms-nl_NL.po
wp-content/languages/plugins/list-field-number-format-for-gravity-forms-nl_NL.mo

You can experience some problems with this plugin when you use it in the European notation or Euro currency. There is a work-around for that. Not perfect, but it solves the problem for now. Just add this to your funtions.php file in your theme:

add_filter( 'gform_currencies', 'update_currency' );
function update_currency( $currencies ) {
    $currencies['EUR'] = array(
        'name'               => __( 'Euro', 'gravityforms' ),
        'symbol_left'        => '€',
        'symbol_right'       => '',
        'symbol_padding'     => ' ',
        'thousand_separator' => ' ',
        'decimal_separator'  => ',',
        'decimals'           => 0
    );

    return $currencies;
}

It makes the euro notation without decimals and it uses a ‘space’ as thousands separator in stead of a comma or point. This makes all the calculations work perfectly.